If there is a trailing empty line in a deeper level (say: 4), TP removes tabs from that line when opening the document. Checking the closed document with BBEdit shows the tabs still in place.
ex 1.taskpaper (47 Bytes)
originally saved like this:
the re-opened file:
this is how I use this in real life:
collapsed it looks like this:
when I reopen the file, tabs are being removed and it looks like this: